This invention relates to an improved process for preparing high molecular weight linear polyesters and more particularly to a process of improving the rate of polycondensation in the production of polyester material.
The preparation of high molecular weight linear polyester material is normally by two reaction stages. In the preparation of polyethylene terephthalate, for example, the first stage consists of the formation of a prepolymer, bis-(2-hydroxyethyl) terephthalate, either by an ester interchange reaction between ethylene glycol and dimethyl terephthalate or by direct esterification of ethylene glycol with terephthalic acid. In the second stage the prepolymer is polycondensed at elevated temperatures under reduced pressure to form the polyester, polyethylene terephthalate.
Typically, various catalysts have been employed to aid both the formation of the prepolymer and the rate of polycondensation. Examples of such catalysts include metal alkanoates, e.g., manganese acetate and cobalt acetate as well as transition metal compounds, e.g., antimony acetate, antimony trioxide and titanium alkoxides. The use of certain phosphorous compounds is also known to be advantageous in the preparation of polyester material, particularly to improve selected physical properties of the polyester material. For example, U.S. Pat. No. 3,028,366 teaches that by adding small amounts of certain phosphorous containing compounds, such as phosphoric acid, alkyl-, hydroxyalkyl- or aryl-phosphates, to the prepolymer, and then polycondensing in the presence of a metal-containing catalyst, substantially colorless polymer can be obtained. U.S. Pat. No. 3,441,540 discloses the use of polymeric phosphoric and/or phosphorous esters of polyhydric alcohols during the polycondensing stage to obtain polymers having improved thermal stability and whiteness. See also Kamatani et al, Polymer Journal, Vol. 12, No. 2, pp. 125-130 (1980), who report the polycondensation of bis(-2-hydroxyethyl)terephthalate in the presence of antimony acetate and phosphoric acid.
An improved process for producing high molecular weight linear polyester material has now been found. Surprisingly, it has been found that in polyester processes which employ selected transesterification and polycondensation catalysts the polycondensation rate can be increased by delaying the addition of known phosphorous-containing compounds until after the prepolymer has first been polycondensed to a certain degree of polymerization.
The process of the present invention requires that the phosphorous containing compound be added after only a short period of polycondensation relative to the entire period of polycondensation. By the process of the present invention, polyesters such as polyethylene terephthalate having good physical properties are produced in less time and thus more economically than previously since polycondensation to a desired molecular weight can be achieved within a shorter period of time.